Pressure-sensitive adhesive composition and pressure-sensitive adhesive sheet

ABSTRACT

The PSA composition provided by the present invention comprises a polymer A and a polymer B different from the polymer A. In each of the polymer A and the polymer B, isobutylene is polymerized at a ratio of 50% by weight or higher.

TECHNICAL FIELD

The present invention relates to a pressure-sensitive adhesivecomposition and a pressure-sensitive adhesive sheet.

The present invention claims priority to Japanese Patent ApplicationsNo. 2017-253955 filed on Dec. 28, 2017 and No. 2018-114937 filed on Jun.15, 2018; and the entire content thereof is incorporated herein byreference.

BACKGROUND ART

In general, pressure-sensitive adhesive (PSA) exists as a soft solid (aviscoelastic material) in a room temperature range and has a property toadhere easily to an adherend with some pressure applied. For such aproperty, PSA is widely used in a form of, for instance, an on-substratePSA sheet having a PSA layer on a support substrate, for purposes suchas bonding, fastening, protection and sealing in various applicationssuch as electronic devices. For instance, technical literatures relatedto PSA sheets that airtightly seal internal spaces of magnetic discdevices include Patent Documents 1 to 3. In this application, becausethe allowable maximum temperature is limited, PSA that does not requireheat for press-bonding is preferably used as the bonding means.

CITATION LIST Patent Literature

[Patent Document 1] Japanese Patent Application Publication No.2014-162874

[Patent Document 2] Japanese Patent Application Publication No.2017-014478

[Patent Document 3] Japanese Patent Application Publication No.2017-160417

SUMMARY OF INVENTION Technical Problem

For instance, the conventional PSA sheets all comprise non-breathablesubstrates and are used in magnetic disc devices such as hard discdrives (HDD), in embodiments to seal their internal spaces wheremagnetic discs (typically HD) are contained. In particular, a void spacethat can be present between a cover member and a housing base member inwhich the magnetic disc is placed can be covered and sealed with a PSAsheet so as to obtain airtightness for the internal space of the device.Such airtight properties may be essential and particularly important ina type of device whose internal space is filled with a low-density gassuch as helium in order to reduce the influence of air flow generated bythe spinning disc. In an embodiment using the PSA sheet, the sealingstructure can be made thinner than in a conventional magnetic discdevice for which airtightness has been assured with a gasket; andtherefore, this embodiment is advantageous in increasing the density andcapacity of a magnetic disc device. This embodiment does not require useof a liquid gasket. Thus, it can mitigate outgassing (gas emission)problems due to gasket.

Lately, studies are underway on magnetic disc devices using HAMR(heat-assisted magnetic recording) for further increases in capacity. Inshort, HAMR is a technology that uses a laser beam to increase theirsurface recording densities. In this technology, the presence ofinternal moisture attenuates the laser beam and badly impacts on therecording life (the number of times that it can be overwritten). Thus,it is desirable to exclude moisture as much as possible. About thisaspect, in Patent Documents 2 and 3, cup methods are used to testmoisture permeability of PSA sheets having aluminum layers. From thestandpoint of producing higher-capacity, higher-quality magnetic discdevices, greater moisture resistance is required of a PSA sheet used inthis application.

The present invention has been made in view of these circumstances withan objective to provide a PSA composition capable of bringing aboutexcellent moisture resistance. Another related objective of thisinvention is to provide a PSA sheet.

Solution to Problem

The present description provides a PSA composition comprising a polymerA and a polymer B different from the polymer A. In the PSA composition,in each of the polymer A and the polymer B, isobutylene is polymerizedat a ratio of 50% by weight or higher (i.e. the polymer A and thepolymer B are individually formed with at least 50% (by weight)polymerized isobutylene). With the PSA composition, it is possible toobtain a PSA sheet having excellent moisture resistance. A typicalexample of the polymer A is polyisobutylene.

In a preferable embodiment of the art disclosed herein (including PSAcompositions, PSA sheets, magnetic disc devices and others; the sameapplies, hereinafter), in addition to the isobutylene, isoprene iscopolymerized in the polymer B. With the use of the polymer B, theeffect of the art disclosed herein is preferably obtained. A typicalexample of the polymer B is butyl rubber.

In a preferable embodiment of the art disclosed herein, the polymer Ahas a weight average molecular weight in the range between 1×10⁴ and80×10⁴. According to an embodiment that includes a polymer A having aweight average molecular weight (Mw) in this range, moisture resistanceand adhesive properties can be preferably combined.

In a preferable embodiment of the art disclosed herein, the polymer Bhas a weight average molecular weight in the range between 5×10⁴ and150×10⁴. According to an embodiment that includes a polymer B having aMw in this range, moisture resistance and adhesive properties can bepreferably combined.

In a preferable embodiment of the art disclosed herein, the ratio(M_(B)/M_(A)) of the polymer B's weight average molecular weight M_(B)to the polymer A's weight average molecular weight M_(A) has a value inthe range between 5 and 100. With the combined use of polymers A and Bsatisfying the M_(B)/M_(A) ratio value range, moisture resistance andadhesive properties can be well balanced with improvements.

In a preferable embodiment of the art disclosed herein, the ratio(C_(A)/C_(B)) of the polymer A content C_(A) to the polymer B contentC_(B) is in the range between 70/30 and 30/70. When it has a compositionsatisfying the C_(A)/C_(B) weight ratio, greater moisture resistance canbe obtained.

In a preferable embodiment of the art disclosed herein, the combinedamount of the polymer A and the polymer B accounts for 90% by weight ormore of the solid content (non-volatiles) of the PSA composition. Whenit has such a composition, the effect of the art disclosed herein ispreferably obtained.

The PSA composition disclosed herein may form a PSA having excellentmoisture resistance; and therefore, it is preferably used for sealing aninternal space of a magnetic disc device for which entry of moistureneeds to be limited.

The present description also provides a PSA sheet having a PSA layercomprising a polymer A and a polymer B different from the polymer A. Ineach of the polymer A and the polymer B in the PSA layer, isobutylene ispolymerized at a ratio of 50% by weight or above. According to the PSAsheet, excellent moisture resistance is obtained.

In a preferable embodiment, the PSA sheet disclosed herein has amoisture permeability of 90 μg/cm²·24 h in in-plane direction of bondinginterface, determined at a permeation distance of 2.5 mm based on aMOCON method. The PSA sheet satisfying this property has excellentmoisture resistance. Thus, it can be preferably used in an applicationfor which the presence of moisture and volatile gas is not desirable.For instance, when the PSA sheet disclosed herein is used as a sealingmaterial in a magnetic disc device, it is possible to greatly limitchanges (typically increases) in internal humidity that may affect thenormal and highly precise operation of the device.

The PSA sheet according to a preferable embodiment has an amount ofthermally released gas of 10 μg/cm² or less, determined at 130° C. for30 minutes by gas chromatography/mass spectrometry (GC-MS). The amountof gas thermally released by the PSA sheet is highly limited as well;and therefore, it can be preferably used in an application for which thepresence of moisture and volatile gas is undesirable. For instance, whenthe PSA sheet disclosed herein is used as a sealing material in amagnetic disc device, it is possible to greatly limit changes (typicallyincreases) in internal humidity that may affect the normal and highlyprecise operation of the device as well as introduction of gas (siloxanegas, etc.) into the system.

The PSA sheet according to a preferable embodiment has a 180° peelstrength (adhesive strength) of 3 N/20 mm or greater to a stainlesssteel plate. For instance, when the PSA sheet is used to seal theinternal space of a sort of magnetic disc device, the PSA sheet havingsuch adhesive strength can adhere well to the adherend, providing goodsealing properties.

In a preferable embodiment of the PSA sheet disclosed herein, the PSAlayer has a storage modulus G′(25° C.) less than 0.5 MPa (morespecifically 0.09 MPa or greater and 0.29 MPa or less) at 25° C. Withthe use of the PSA layer having a storage modulus G′(25° C.) of at leastthe prescribed value (more specifically in the prescribed range), thePSA layer is highly wet and tightly adheres to the adherend's surface,whereby excellent moisture resistance is likely to be obtained.

The PSA sheet according to a preferable embodiment shows a displacementless than 2 mm in a shear holding power test carried out with a 1 kgload at 60° C. for one hour. The PSA sheet satisfying this property canprovide good holding power even when used at a relatively hightemperature.

The PSA sheet disclosed herein has excellent moisture resistance. Thus,it is preferably used for sealing the internal space of a magnetic discdevice where entry of moisture needs to be limited. The art disclosedherein provides a magnetic disc device comprising a PSA sheet disclosedherein. The PSA sheet may serve to seal the internal space of themagnetic disc device. In the magnetic disc device in such an embodiment,the PSA sheet is relatively thin, yet provides moisture resistance andairtight properties; and therefore, as compared to a conventionalgasket-type product, the capacity can be further increased. Inparticular, with the use of the PSA sheet disclosed herein in a HAMRmagnetic disc device, a magnetic recording device having a higherdensity can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional diagram schematically illustrating aconstitutional example of the PSA sheet.

FIG. 2 shows a schematic diagram of the moisture permeabilitymeasurement method.

FIG. 3 shows an enlarged top view of a sample used in determining themoisture permeability.

FIG. 4 shows a cross-sectional diagram schematically illustrating themagnetic disc device according to an embodiment.

FIG. 5 shows a cross-sectional diagram schematically illustrating themagnetic disc device according to another embodiment.

DESCRIPTION OF EMBODIMENTS

Preferable embodiments of the present invention are described below.Matters necessary to practice this invention other than thosespecifically referred to in this description can be understood by aperson skilled in the art based on the disclosure about implementing theinvention in this description and common technical knowledge at the timethe application was filed. The present invention can be practiced basedon the contents disclosed in this description and common technicalknowledge in the subject field. In the drawings referenced below, acommon reference numeral may be assigned to members or sites producingthe same effects, and redundant descriptions are sometimes omitted orsimplified. The embodiments described in the drawings are schematizedfor clear illustration of the present invention, and do not necessarilyrepresent the accurate size or reduction scale of an actual product ofthe PSA sheet or magnetic disc device of this invention or of themoisture permeability measurement device.

As used herein, the term “PSA” refers to, as described earlier, amaterial that exists as a soft solid (a viscoelastic material) in a roomtemperature range and has a property to adhere easily to an adherendwith some pressure applied. As defined in “Adhesion: Fundamentals andPractice” by C. A. Dahlquist (McLaren & Sons (1966), P. 143), ingeneral, PSA referred to herein can be a material that has a propertysatisfying complex tensile modulus E* (1 Hz)<10⁷ dyne/cm² (typically, amaterial that exhibits the described characteristics at 25° C.).

The concept of PSA sheet herein may encompass so-called PSA tape, PSAlabels, PSA film, etc. The PSA sheet disclosed herein can be in a rollor in a flat sheet. Alternatively, the PSA sheet may be processed intovarious shapes.

<Constitution of PSA Sheet>

The PSA sheet disclosed herein can be, for instance, an adhesivelysingle-faced PSA sheet having a cross-sectional structure as shown inFIG. 1. A PSA sheet 1 comprises a substrate layer 10 and a PSA layer 20supported on a first face of substrate layer 10. In particular,substrate layer 10 is a layered body (laminate film) in which a firstresin layer 12, an inorganic layer 14 and a second resin layer 16 arelayered in this order; and it is a moisture-impermeable layer that isimpermeable to moisture in the thickness direction. The first resinlayer 12 placed on the first face side of inorganic layer 14 forms anouter surface of PSA sheet 1 while the second resin layer 16 is placedon the second face side of inorganic layer 14, that is, the PSA layer 20side. From the standpoint of the moisture resistance, PSA layer 20 isformed continuously over the entire first face of substrate layer 10 atleast in the area that bonds to an adherend. PSA sheet 1 prior to use(before applied to the adherend) may be protected with a release liner(not shown in the drawing) having a release face at least on the PSAlayer 20 side surface.

<PSA Composition> (Polymer A)

The PSA composition (and even the PSA layer; the same applieshereinafter unless otherwise noted) comprises a polymer A. The polymer Ais an isobutylene-based polymer in which an isobutylene is polymerized,accounting for 50% by weight or more thereof. As used herein, the term“isobutylene-based polymer” is not limited to isobutylene homopolymer(homo-isobutylene); it also encompasses a copolymer whose primarymonomer is isobutylene (a copolymer primarily formed of isobutylene).Due to their molecular structures, isobutylene-based polymers are highlyhydrophobic and their main chains have low mobility. Thus, a PSA layerformed from a PSA composition comprising an isobutylene-based polymermay have relatively low moisture permeability on its own. This isadvantageous from the standpoint of preventing water vapor frompermeating through the lateral surface of the PSA layer at an edges faceof the PSA sheet. Such a PSA layer tends to have a good elastic modulusas well as excellent removability. Specific examples of theisobutylene-based polymer include polyisobutylene andisobutylene-isoprene copolymer (butyl rubber).

The starting monomer mixture to form the polymer A disclosed hereininclude isobutylene accounting for 50% or more by weight (e.g. more than50% by weight), preferably 75% or more by weight, more preferably 85% ormore by weight, or yet more preferably 90% or more by weight (e.g. 95%by weight or more) thereof. The ratio of isobutylene in the entirestarting monomer mixture can be 99% to 100% by weight. Theisobutylene-based polymer can be a copolymer in which isobutyleneaccounts for more than 50% by weight of the monomers, or even 70% byweight or more. Examples of the copolymer include a copolymer ofisobutylene and butene (normal butylene), a copolymer (butyl rubber) ofisobutylene and isoprene, vulcanized products and modified products ofthese. Examples of the copolymers include butyl rubbers such as regularbutyl rubber, chlorinated butyl rubber, brominated butyl rubber, andpartially crosslinked butyl rubber. Examples of the vulcanized andmodified products include those modified with functional groups such ashydroxy group, carboxy group, amino group, and epoxy group. Theisobutylene-based polymer that can be preferably used from thestandpoint of the moisture resistance, reduction of outgassing, andadhesive strength, etc., includes polyisobutylene andisobutylene-isoprene copolymer (butyl rubber). The copolymer can be acopolymer (e.g. an isobutylene-isoprene copolymer) of which the othermonomers (isoprene, etc.) excluding isobutylene has a copolymerizationratio lower than 30% by mol.

As used herein, the “polyisobutylene” refers to a polyisobutylene inwhich the copolymerization ratio of monomers excluding isobutylene is10% or lower (preferably 5% or lower) by weight. In particular,homo-isobutylene is preferable. As the polymer A, several species ofisobutylene-based polymer (typically polyisobutylene) with varied Mwvalues can be used together.

In addition to isobutylene, the starting monomer mixture to form thepolymer A disclosed herein may optionally include one, two or morespecies of monomers (non-isobutylene monomers) selected among butene,isoprene, butadiene, styrene, ethylene and propylene. The polymer A canbe a copolymer obtainable by copolymerizing one, two or more species ofthe examples of monomers. The starting monomer mixtures for forming thepolymer A disclosed herein typically comprises one, two or more speciesof non-isobutylene monomers at a ratio of 50% or below (e.g. below 50%by weight), preferably about 25% or below, more preferably about 15% orbelow, or yet more preferably about 10% or below (e.g. about 5% orbelow). The non-isobutylene monomer content in the entire startingmonomer mixture can also be about 1% by weight or less. The polymer Aaccording to a preferable embodiment is a copolymer obtainable bycopolymerizing a monomer selected among isoprene and butenes as thenon-isobutylene monomer. From the standpoint of reduction of outgassing(especially, reduction of gas formation that may degrade the durability,reliability or accurate operation of electronic devices such as magneticdisc devices), the styrene content in the starting monomer mixture ispreferably less than 10% by weight or more preferably less than 1% byweight. The art disclosed herein can be preferably implemented in anembodiment where the starting monomer mixture is essentially free ofstyrene.

The molecular weight of the polymer A (typically, polyisobutylene) isnot particularly limited. For instance, a species having a Mw of about1×10⁴ or higher can be suitably selected and used. The maximum Mw is notparticularly limited and can be about 150×10⁴ or lower. From thestandpoint of the moisture resistance, the polymer A according to apreferable embodiment has a Mw of preferably about 100×10⁴ or lower, forinstance, about 80×10⁴ or lower. From the standpoint of the PSAs elasticmodulus, cohesive strength and so on, the Mw is preferably about 2×10⁴or higher, more preferably about 3×10⁴ or higher, or yet more preferablyabout 5×10⁴ or higher (e.g. about 7×10⁴ or higher). From the standpointof the moisture resistance, the Mw is preferably about 50×10⁴ or lower,more preferably about 30×10⁴ or lower, yet more preferably about 15×10⁴or lower, or particularly preferably about 10×10⁴ or lower (e.g. below10×10⁴). The polymer A according to another embodiment may have a Mw of,for instance, about 5×10⁴ or higher, or preferably about 15×10⁴ orhigher (typically about 30×10⁴ or higher).

While no particular limitations are imposed, as the polymer A(typically, polyisobutylene), it is preferable to use a species having adispersity (Mw/Mn) (which is indicated as a ratio of weight averagemolecular weight (Mw) to number average molecular weight (Mn)) in arange of 3 to 7 (more preferably 3 to 6, e.g. 3.5 to 5.5). For instance,as the polymer A, several species of polyisobutylene varying in Mw/Mncan be used together.

The Mw and Mn values of polymer A here refer to values based onpolystyrene that are determined by gel permeation chromatography (GPC)analysis. As the GPC analyzer, for instance, model name HLC-8120 GPCavailable from Tosoh Corporation can be used. The Mw and Mn of polymer B(e.g. butyl rubber) can also be determined by similar GPC analysis.

(Polymer B)

In addition to the polymer A, the PSA composition disclosed hereincomprises a polymer B different from the polymer A. Just like theaforementioned polymer A, the polymer B is an isobutylene-based polymerin which isobutylene is polymerized at a ratio of at least 50% byweight, but has a monomer composition different from that of the polymerA. In typical, the polymers A and B are different species of polymer.

The polymer B is typically a copolymer in which isobutylene accounts formore than 50% by weight, or even 70% by weight or more of the monomerstherein. The starting monomer mixture for forming the polymer Bdisclosed herein includes about 60% (by weight) isobutylene or more,preferably about 70% by weight or more, more preferably about 80% byweight or more, or yet more preferably about 90% by weight or more (e.g.about 95% by weight or more). The copolymer can be, for instance, acopolymer of isobutylene and butene (normal butylene), a copolymer ofisobutylene and isoprene (i.e. butyl rubber), vulcanization ormodification products of these, etc. Examples of the copolymer includebutyl rubbers such as regular butyl rubber, chlorinated butyl rubber,brominated butyl rubber, and partially-crosslinked butyl rubber.Examples of the vulcanization and modification products include speciesmodified with functional groups such as hydroxy group, carboxy group,amino group and epoxy group. Isobutylene-based polymers that can bepreferably used from the standpoint of the moisture resistance,reduction of outgassing, the adhesive strength, etc., includepolyisobutylene and isobutylene-isoprene copolymer (butyl rubber). Sucha copolymer can be, for instance, a copolymer (e.g. isobutylene-isoprenecopolymer) in which non-isobutylene monomers (isoprene, etc.) has acopolymerization ratio below 30% by mol.

In a preferable embodiment, the polymer B is a polymer in whichisobutylene and isoprene are copolymerized, typically anisobutylene-isoprene copolymer (butyl rubber). In the copolymer, thecombined amount of isobutylene and isoprene as monomers accounts fortypically at least 50% (e.g. at least 70%, preferably at least 80%, oryet more preferably at least 90%) by weight of the entire monomers. In aparticularly preferable embodiment, the combined amount of isobutyleneand isoprene as monomers accounts for about 95% by weight or more (e.g.99% to 100% by weight) of all monomers.

In addition to isobutylene, the starting monomer mixture for forming thepolymer B disclosed herein may include one, two or more species ofmonomers (non-isobutylene monomers) optionally selected among butene,isoprene, butadiene, styrene, ethylene and propylene. The polymer B canbe a copolymer obtainable by copolymerizing one, two or more species ofthe exemplified monomers. The starting monomer mixture for forming thepolymer B disclosed herein typically includes one, two or more speciesof non-isobutylene monomers at a ratio of 50% by weight or below (e.g.below 50% by weight). The non-isobutylene monomer content can be, forinstance, about 25% by weight or less, about 15% by weight or less, oreven about 10% by weight or less (e.g. about 5% by weight or less).

In an embodiment using a butyl rubber as the polymer B, the ratio ofisoprene as a monomer of the polymer B is below 50% by weight, forinstance, suitably about 40% by weight or below, preferably about 30% byweight or below, more preferably about 20% by weight or below, or yetmore preferably about 10% by weight or below (e.g. about 5% by weight orbelow).

From the standpoint of reduction of outgassing (especially, reduction ofgas formation that may degrade the durability, reliability or accurateoperation of electronic devices such as magnetic disc devices), thestyrene content in the monomers is preferably below 10% by weight, ormore preferably below 1% by weight. The art disclosed herein can bepreferably implemented in an embodiment where the starting monomermixture is essentially free of styrene.

The molecular weight of the polymer B (e.g. butyl rubber) is notparticularly limited. For instance, a species having a Mw in a rangebetween 5×10⁴ and 100×10⁴ can be suitably selected and used. In view ofthe balance between the ease of forming the PSA layer and tight adhesion(adhesive strength) to adherend, the butyl rubber has a Mw of preferably10×10⁴ or higher, more preferably 15×10⁴ or higher, or yet morepreferably about 30×10⁴ or higher (e.g. 50×10⁴ or higher); and suitablyabout 150×10⁴ or lower, preferably 100×10⁴ or lower, more preferably80×10⁴ or lower, or yet more preferably about 70×10⁴ or lower (e.g.about 60×10⁴ or lower). Several species of butyl rubber varying in Mwcan be used together.

While no particular limitations are imposed, the butyl rubber has adispersity (Mw/Mn) in a range between 3 and 8 or more preferably in arange between 4 and 7. When using butyl rubber as the polymer B, severalspecies of butyl rubber varying in Mw/Mn can be used together.

The Mooney viscosity of the butyl rubber is not particularly limited.For instance, a butyl rubber having a Mooney viscosity ML₁₊₈(125° C.)between 10 and 100 can be used. In view of the balance between the PSAlayer's ease of formation and tightness of bonding to adherend (adhesivestrength), a butyl rubber having a Mooney viscosity ML₁₊₈(125° C.) of 15to 80 (more preferably 30 to 70, e.g. 40 to 60) is preferable.

In the embodiment using polymers A and B together, because they vary inmolecular weight, it is possible to preferably bring about moistureresistance based on the lower molecular polymer as well as adhesiveproperties (cohesive strength, etc.) based on the higher molecularweight polymer. From such a standpoint, in an embodiment in which thepolymer A has a relatively higher molecular weight, the ratio(M_(A)/M_(B)) of polymer A's Mw (M_(A)) to polymer B's Mw (M_(B)) ishigher than 1, preferably about 2 or higher, more preferably about 3 orhigher, or yet more preferably about 5 or higher (e.g. about 7 orhigher). The maximum M_(A)/M_(B) ratio value is suitably about 100 orlower, preferably about 50 or lower, more preferably about 20 or lower,or yet more preferably about 10 or lower (e.g. lower than 10). In anembodiment in which the polymer B has a relatively higher molecularweight, the ratio (M_(B)/M_(A)) of polymer B's Mw (M_(B)) to polymer A'sMw (M_(A)) is higher than 1, preferably about 2 or higher, morepreferably about 3 or higher, or yet more preferably about 5 or higher(e.g. about 7 or higher). The maximum M_(B)/M_(A) ratio value issuitably about 100 or lower, preferably about 50 or lower, morepreferably about 20 or lower, or yet more preferably about 10 or lower(e.g. lower than 10).

The blend ratio of A to B can be suitably selected so as to obtainpreferable elastic modulus, moisture resistance and adhesive propertiesdisclosed herein. The weight ratio (C_(A)/C_(B)) of polymer A content(C_(A)) to polymer B content (C_(B)) can be, for instance, 95/5 to 5/95,preferably 90/10 to 10/90, more preferably 80/20 to 20/80, yet morepreferably 70/30 to 30/70, or particularly preferably 60/40 to 40/60(typically 55/45 to 45/55).

In a preferable embodiment, the dispersity (Mw/Mn) of the polymers atlarge in the PSA composition is 3 or higher, or more preferably 4 orhigher. According to the PSA comprising such polymers, adhesive strengthcan be easily combined with resistance to leftover adhesive residue. Italso brings the PSA layer's elastic modulus in a favorable range andgood moisture resistance tends to be obtained. At or above a certainMw/Mn value, the PSA can be obtained with a low solution viscosity forits Mw. The dispersity of the polymers at large can also be 5 or higher,6 or higher, or even 7 or higher. The maximum dispersity of the polymersat large is not particularly limited; it is preferably 10 or lower (e.g.8 or lower).

The combined ratio of polymers A and B in the solid content (possibly aPSA layer) of the PSA composition disclosed herein is not particularlylimited. It is usually about 50% by weight or higher, or suitably about80% by weight or higher. In a preferable embodiment, the combined ratioof polymers A and B is about 90% by weight or higher, more preferablyabout 95% by weight or higher, or yet more preferably about 97% byweight or higher (e.g. 99% to 100% by weight). The solid content of thePSA composition and a PSA layer formed from the PSA composition mayessentially consist of polymers A and B.

The art disclosed herein can be preferably implemented in an embodimenthaving a PSA layer formed of a PSA (a non-crosslinked PSA) in which thepolymers are not crosslinked. Here, the term “PSA layer formed of anon-crosslinked PSA” refers to a PSA layer that has not been subjectedto an intentional treatment (i.e. crosslinking treatment, e.g. additionof a crosslinking agent, etc.) for forming chemical bonds among thepolymers.

(Other Additives)

The PSA composition may comprise, as necessary, various additivesgenerally used in the PSA field, such as tackifier (tackifier resin),leveling agent, crosslinking accelerator, plasticizer, fillers,colorants including pigments and dyes, softening agent, anti-staticagent, anti-aging agent, UV-absorbing agent, antioxidant andphoto-stabilizing agent. Optionally, a third polymer that is neitherpolymer A nor B may be included as well. With respect to these variousadditives, heretofore known species can be used by typical methods. Fromthe standpoint of avoiding the use of a low-molecular-weight componentwhich may be susceptible to outgassing, the other additive content (e.g.tackifier resin, anti-aging agent, UV absorber, antioxidant,photo-stabilizer) in the PSA composition is preferably limited to orbelow about 10% by weight (e.g. to or below 5% by weight, typically toor below 3% by weight). The art disclosed herein can be preferablyimplemented in an embodiment where the PSA composition is essentiallyfree of other additives (e.g. tackifier resin, anti-aging agent, UVabsorber, antioxidant, photo-stabilizer).

The form of the PSA composition is not particularly limited. Forinstance, it can be in various forms, such as a PSA composition (asolvent-based PSA composition) that comprises PSA-layer-formingmaterials as described above in an organic solvent, a PSA composition(water-dispersed PSA composition, typically an aqueous emulsion-basedPSA composition) in which the PSA is dispersed in an aqueous solvent, aPSA composition that is curable by an active energy ray (e.g. UV ray),and a hot-melt PSA composition. From the standpoint of the ease ofapplication and the adhesive properties, a solvent-based PSA compositioncan be preferably used. As the solvent, it is possible to use onespecies of solvent or a mixture of two or more species, selected amongaromatic compounds (typically aromatic hydrocarbons) such as toluene andxylene; acetic acid esters such as ethyl acetate and butyl acetate; andaliphatic or alicyclic hydrocarbons such as hexane, cyclohexane, heptaneand methyl cyclohexane. While no particular limitations are imposed, itis usually suitable to adjust the solvent-based PSA composition toinclude 5% to 30% non-volatiles (NV) by weight. Too low an NV tends toresult in higher production costs while too high an NV may degrade thehandling properties such as the ease of application.

<PSA Layer>

The PSA layer in the art disclosed herein can be formed using the PSAcomposition in accordance with a known method for forming PSA layers inPSA sheets. For instance, it is preferable to employ a direct methodwhere the PSA composition with PSA layer-forming materials as describedabove dissolved or dispersed in a suitable solvent is directly provided(typically applied) to a substrate and allowed to dry to form a PSAlayer. Alternatively, it is also possible to employ a transfer methodwhere the PSA composition is provided to a releasable surface (e.g. asurface of a release liner, a substrate's backside treated with releaseagent, etc.) and allowed to dry to form a PSA layer on the surface, andthe PSA layer is transferred to a non-releasable substrate. As therelease face, a surface of a release liner, a substrate's back face thathas been treated with release agent, and the like can be used. The PSAlayer disclosed herein is typically formed in a continuous manner.

The PSA composition can be applied, for instance, with a known orcommonly used coater such as gravure roll coater, reverse roll water,kiss roll water, dip roll coater, bar coater, knife coater, and spraywater.

In the art disclosed herein, the thickness of the PSA layer forming theadhesive face is not particularly limited. The PSA layer has a thicknessof suitably 3 am or greater, preferably 10 am or greater, or morepreferably 20 am or greater. With increasing thickness of the PSA layer,the adhesive strength to adherend tends to increase. Having at least acertain thickness, the PSA layer absorbs the adherend's surfaceroughness to form tight adhesion. When the PSA layer has a thickness of10 μm or greater, for instance, it can provide good, tight adhesion toan adherend having a surface whose arithmetic mean surface roughness Rais about 1 μm to 5 μm (e.g. 3 μm). The thickness of the PSA layerforming the adhesive face can be, for instance, 150 μm or less; it issuitably 100 μm or less, or preferably 50 μm or less. With decreasingthickness of the PSA layer, it tends to show a greater ability toinhibit water vapor from laterally permeating the PSA layer, leading toreduction of outgassing from the PSA layer. A smaller thickness of thePSA layer is also advantageous from the standpoint of reducing thethickness and weight of the PSA sheet.

The storage modulus at 25° C., G′(25° C.), of the PSA layer disclosedherein is not particularly limited and it can be set in a suitable rangeaccording to required properties, etc. In a preferable embodiment, theG′(25° C.) is less than 0.5 MPa. The PSA layer whose G′(25° C.) is at orbelow a prescribed value wets the adherend surface well to form tightadhesion. The G′(25° C.) is more preferably 0.4 MPa or less, or yet morepreferably 0.3 MPa or less (e.g. 0.25 MPa or less). The G′(25° C.) canalso be, for instance, 0.2 MPa or less. The G′(25° C.) value is notparticularly limited and is suitably greater than about 0.01 MPa. Fromthe standpoint of the adhesive properties, prevention of leftoveradhesive residue, etc., it is preferably 0.05 MPa or greater, or morepreferably 0.07 MPa or greater (e.g. 0.1 MPa or greater). In aparticularly preferable embodiment, the PSA layer has a storage modulusG′(25° C.) of 0.09 MPa or greater (e.g. 0.16 MPa or greater) and 0.29MPa or less (e.g. 0.24 MPa or less). When the PSA layer has a storagemodulus G′(25° C.) in these ranges, excellent moisture resistance can beobtained.

In the art disclosed herein, the storage modulus G′(25° C.) of a PSAlayer can be determined by dynamic elastic modulus measurement. Inparticular, several layers of the PSA subject to measurement are layeredto fabricate an approximately 2 mm thick PSA layer. A specimen obtainedby punching out a disc of 7.9 mm diameter from the PSA layer is fixedbetween parallel plates. With a rheometer (e.g. ARES available from TAInstruments or a comparable system), dynamic elastic modulus measurementis carried out to determine storage modulus G′(25° C.). The PSA layersubject to measurement can be formed by applying a layer thecorresponding PSA composition on a release face of a release liner orthe like and allowing it to dry or cure. The thickness (coatingthickness) of the PSA layer subjected to the measurement is notparticularly limited as long as it is 2 mm or less. It can be, forinstance, about 50 μm.

-   -   Measurement mode: shear mode    -   Temperature range: −50° C. to 150° C.    -   Heating rate: 5° C./min    -   Measurement frequency: 1 Hz

The same measurement method is also used in the working examplesdescribed later.

<Substrate Layer>

The PSA sheet disclosed herein may typically has a substrate layer.There are no particular limitations to the substrate layer-formingmaterial that can be used in the art disclosed herein. As the substratelayer, in accordance with the purpose of the PSA sheet, a suitablespecies can be selected and used among, for instance, plastic film suchas polypropylene film, ethylene-propylene copolymer film, polyester filmand polyvinyl chloride film; a sheet formed of foam such as polyurethanefoam, polyethylene foam and polychloroprene foam; woven fabrics andnonwoven fabrics (including paper such as Japanese paper and high-gradepaper) of pure or blended yarn of various fibrous materials (possiblynatural fibers such as hemp and cotton, synthetic fibers such aspolyester and vinylon, semi-synthetic fibers such as acetate, etc.); andmetal foil such as aluminum foil and copper foil.

The substrate layer according to a preferable embodiment is amoisture-impermeable layer. As used herein, the moisture-impermeablelayer refers to a layer (film) that has a moisture permeability (a watervapor transmission rate in the thickness direction) of less than 5×10⁻¹g/m²·24 h when determined at 40° C. at 90% RH based on the MOCON method(JIS K7129:2008). With the use of the moisture-impermeable layersatisfying this property, it is possible to obtain a PSA sheet havingmoisture resistance in the thickness direction. The moisturepermeability is preferably less than 5×10⁻² g/m²·24 h, or morepreferably less than 5×10⁻³ g/m²·24 h, for instance, less than 5×10⁻⁵g/m²·24 h. As the moisture permeability measurement device,PERMATRAN-W3/33 available from MOCON, Inc. or a comparable product canbe used.

In a preferable embodiment, the substrate layer disclosed hereinincludes an inorganic layer. The material or structure of the inorganiclayer is not particularly limited and can be selected in accordance ofthe purpose and usage. From the standpoint of the moisture resistanceand airtight properties, it is advantageous that the inorganic layer isessentially non-porous. In typical, a preferable inorganic layer isessentially formed of an inorganic material. For instance, an inorganiclayer formed of at least 95% (by weight) inorganic material ispreferable (more preferably at least 98% by weight, or yet morepreferably at least 99% by weight). The number of inorganic layers inthe substrate layer is not particularly limited; it can be one, two ormore (e.g. about two to five). From the standpoint of the ease ofmanufacturing and availability, the number of inorganic layers in thesubstrate layer is preferably about 1 to 3, or more preferably one ortwo. When the substrate layer includes several inorganic layers, thematerials and structures (thicknesses, etc.) of these inorganic layerscan be the same with or different from one another.

As the inorganic material forming the inorganic layer, it is possible touse, for instance, metal materials including elemental metals such asaluminum, copper, silver, iron, tin, nickel, cobalt, and chromium aswell as alloys of these; and inorganic compounds such as oxides,nitrides and fluorides of metals and metalloids including silicon,aluminum, titanium, zirconium, tin and magnesium. Specific examples ofthe inorganic compounds include silicon oxides (SiO_(x), typicallySiO₂), aluminum oxide (Al₂O₃), silicon nitride (Si₃N₄), silicon oxidenitride (SiO_(x)N_(y)), titanium oxide (TiO₂), and indium tin oxide(ITO).

The metal materials can be used as the inorganic layers as metal foils(e.g. aluminum foil) formed by a known method such as rolling by arolling mill, etc. Alternatively, for instance, a metal material formedin a layer by a known film-forming method such as vacuum vapordeposition, spattering and plating.

The inorganic compound can be typically used as the inorganic layer in aform of thin film formed by a known method. As the method for formingthin film of the inorganic compound, various vapor deposition methodscan be used. For instance, physical vapor deposition methods (PVD) suchas vacuum vapor deposition, spattering and ion plating, chemical vapordeposition methods (CVD) and like method can be used. The substratelayer may further have a resin layer on top of the vapor depositionlayer. For instance, the resin layer may be a topcoat layer provided forpurposes such as protecting the vapor deposition layer.

From the standpoint of the moisture resistance, ease of manufacturing,availability, etc., it is preferable to use an inorganic layer formedof, for instance, aluminum or an aluminum alloy. From the standpoint ofthe moisture resistance, ease of manufacturing, availability, etc., asthe inorganic layer formed of an inorganic compound, for instance, asilicon oxide layer or an aluminum oxide layer can be preferably used.Examples of an inorganic layer preferable for being able to form ahighly transparent inorganic layer include a silicon oxide layer, analuminum oxide layer and an ITO layer.

The maximum thickness of the inorganic layer is not particularlylimited. From the standpoint of obtaining conformability to shapes ofadherends, the inorganic layer advantageously has a thickness of 50 μmor less. From the standpoint of reducing the thickness and weight of thePSA sheet, the thickness of the inorganic layer is suitably 15 am orless, preferably 13 am or less, more preferably 11 am or less, or yetmore preferably 9 μm or less. When the substrate layer includes severalinorganic layers, the combined thickness of these inorganic layers is inthese ranges. The minimum thickness of the inorganic layer is notparticularly limited and can be suitably selected so as to obtain a PSAsheet that shows moisture resistance suited for the purpose and usage.The thickness of the inorganic layer is suitably 1 nm or greater. Fromthe standpoint of the moisture resistance, air-tight properties, etc.,it is preferably 2 nm or greater, or more preferably 5 nm or greater.When the substrate layer includes several inorganic layers, it ispreferable that at least one of them has a thickness in these ranges.Each of the several inorganic layers may have a thickness in theseranges as well.

The preferable thickness range of the inorganic layer may also varydepending on the material of the inorganic layer, the formation method,etc. For instance, when metal foil (e.g. aluminum foil) forms theinorganic layer (or the metal layer), in view of the moistureresistance, ease of manufacturing, crease resistance, etc., itsthickness is suitably 1 am or greater, preferably 2 am or greater, ormore preferably 5 μm or greater. In view of the flexibility which leadsto adherend conformability, the metal layer's thickness is suitably 50μm or less, preferably 20 am or less, more preferably 15 am or less, yetmore preferably 12 μm or less, or particularly preferably 10 μm or less.With respect to the inorganic layer formed by vapor deposition of aninorganic compound, in view of the balance between flexibility and easeof manufacturing the substrate layer, its thickness is suitably in arange between 1 nm and 1000 nm, preferably in a range between 2 nm and300 nm, or more preferably in a range between 5 nm and less than 100 nm.

The substrate layer disclosed herein may include a resin layer inaddition to the inorganic layer. The resin layer may serve as aprotection layer to prevent the inorganic layer from getting damaged bybending deformation and friction. Thus, the substrate layer includingthe resin layer in addition to the inorganic layer is preferable fromthe standpoint of the endurance and reliability of moisture-resistantproperties and also from the standpoint of the ease of handling thesubstrate layer or the PSA sheet. By placing the resin layer on the PSAlayer side surface of the substrate layer, the anchoring of the PSAlayer can be enhanced. When the inorganic layer is formed by vapordeposition, spattering or like method, the resin layer can be used asthe base for forming the inorganic layer.

The structure of the resin layer is not particularly limited. Forinstance, the resin layer may include a void space as in fiberassemblies such as woven fabrics and nonwoven fabrics or as in foambodies such as polyethylene foam; or it can be a resin layer (resinfilm) essentially free of a void space. From the standpoint of reducingthe thickness of the PSA sheet, it is preferable use a resin layeressentially free of a void space.

As the resin material forming the resin layer, it is possible to use,for instance, polyester resins such as polyethylene terephthalate (PET),polybutylene terephthalate (PBT) and polyethylene naphthalate (PEN);polyolefin resins such as polyethylene (PE) and polypropylene (PP);polyimide (PI); polyetheretherketone (PEEK); chlorine-containingpolymers such as polyvinyl chloride (PVC) and polyvinylidene chloride;polyamide-based resins such as nylon and aramid; polyurethane resin;polystyrene-based resin; acrylic resins; fluororesins; cellulose-basedresins; and polycarbonate-based resins. Of these, solely one species ora combination of two or more species can be used. When two or morespecies of resin are used together, these resins can be used blended orseparately. Both thermoplastic resins and thermosetting resins can beused. From the standpoint of the ease of forming film, etc., athermoplastic resin is preferably used.

In the substrate layer including a resin layer, at an edge face of thePSA sheet, water vapor may enter the resin layer from its side (lateralsurface). From the standpoint of inhibiting such entrance of watervapor, as the resin material forming the resin layer, a highlymoisture-resistant material can be preferably used. For instance, apreferable resin layer is formed, using a resin material whose primarycomponent is a polyester resin such as PET or a polyolefinic resin suchas PE and PP. In a preferable embodiment, PET film can be preferablyused as the resin layer. In another preferable embodiment, as the resinlayer, it is preferable to use BOPP (biaxially oriented polypropylene)film obtainable by forming film of a resin material that comprises PP asthe primary component and biaxially stretching the film. In the PSAsheet having no inorganic layer further on the adherend side relative tothe resin layer, it is particularly significant to inhibit entrance ofwater vapor from the lateral surface of the resin layer. A typicalexample of the PSA sheet having such a constitution is a PSA sheet inwhich the PSA layer-side surface of the substrate layer is formed with aresin layer.

The resin layer may include, as necessary, various additives such asfillers (inorganic fillers, organic fillers, etc.), anti-aging agent,antioxidant, UV absorber, anti-static agent, slip agent and plasticizer.The ratio of the various additives included is below about 30% by weight(e.g. below 20% by weight, typically below 10% by weight).

The number of resin layers in the substrate layer is not particularlylimited and it can be one, two or more (e.g. about two to five). Fromthe standpoint of the ease of manufacturing and availability, the numberof resin layers in the substrate layer is preferably one to three, ormore preferably one or two. When the substrate layer includes severalresin layers, the materials and structures (thicknesses, inclusion of avoid space, etc.) of these resin layers can be the same with ordifferent from one another.

The method for forming the resin layer is not particularly limited. Aheretofore known general resin film molding method can be suitablyemployed to form the resin layer, for instance, extrusion molding,inflation molding, T-die casting, calender roll molding and wet casting.The resin layer may a non-stretched kind or may be subjected to astretching process such as uni-axial stretching and bi-axial stretching.

The minimum thickness of the resin layer is not particularly limited.From the standpoint of the crease resistance, ease of forming film,etc., the thickness of the resin layer is suitably 1 μm or greater,preferably 3 μm or greater, more preferably 5 μm or greater, or yet morepreferably 7 μm or greater. When the substrate layer includes severalresin layers, it is preferable that at least one of them has a thicknessin these ranges. Each of the several resin layers may have a thicknessin these ranges as well.

The maximum thickness of the resin layer is not particularly limited.For instance, it can be 100 μm or less. From the standpoint of reducingthe thickness and weight of the PSA sheet, the thickness of the resinlayer is suitably 70 μm or less, preferably 55 μm or less, or morepreferably 35 μm or less. When the substrate layer includes severalresin layers, the combined thickness of these resin layers is preferablyin these ranges. In general, the moisture permeability of the resinlayer is higher than that of the inorganic layer. Thus, it is alsopreferable to make the combined thickness of resin layers smaller fromthe standpoint of preventing water vapor from entering the resin layerfrom its lateral surface.

The inorganic layer and the resin layer are preferably bonded. Thebonding method is not particularly limited. A method known in thepertinent field can be suitably employed. For instance, it is possibleto employ a method (extrusion lamination) where a resin material forforming the resin layer is melted and extruded along with a pre-moldedinorganic layer (typically metal foil), a method where a solution ordispersion of the resin material for forming the resin layer is appliedto a pre-molded inorganic layer and allowed to dry, and like method.Alternatively, it is also possible to employ a method where an inorganiclayer is vapor-deposited on a pre-molded resin layer, a method where aninorganic layer is bonded to a separately-molded resin layer, and likemethod. For instance, the bonding can be achieved by hot pressing. Theresin layer and the inorganic layer can be bonded via an adhesive layeror a PSA layer.

The adhesive layer to bond the resin layer and the inorganic layer canbe an undercoat layer formed by applying an undercoat such as primer tothe resin layer. As the undercoat, those known in the pertinent fieldcan be used, such as urethane-based undercoat, ester-based undercoat,acrylic undercoat, and isocyanate-based undercoat. From the standpointof reducing the thickness and weight of the PSA sheet, the thickness ofthe undercoat layer is suitably 7 μm or less, preferably 5 μm or less,or more preferably 3 μm or less. The minimum thickness of the undercoatlayer is not particularly limited. For instance, it can be 0.01 μm orgreater (typically 0.1 μm or greater).

Before the bonding process, the resin layer may be subjected to commonsurface treatment, chemical or physical treatment, for instance,mattifying treatment, corona discharge treatment, crosslinkingtreatment, chromic acid treatment, ozone exposure, flame exposure,high-voltage electric shock exposure, and ionized radiation treatment.

The PSA layer(s) placed between layers forming the substrate layer tobond them together are not exposed to the surface of the PSA sheet; andtherefore, they do not correspond to the PSA layer forming the adhesiveface of the PSA sheet. In the PSA sheet disclosed herein, the materialand physical properties of such a PSA layer for internal use in thesubstrate layer are not particularly limited. The PSA layer can beformed of a PSA similar to the PSA layer forming the adhesive face orcan be formed of a different PSA. It is not particularly limited inthickness, either. For instance, it may have a comparable thickness tothe undercoat layer.

Favorable examples of the substrate layer used in the PSA sheetdisclosed herein include a substrate layer formed of a laminate bodythat comprises an inorganic layer as well as first and second resinlayers laminated on top and bottom of the inorganic layer. The first andsecond resin layers forming the substrate layer are laminated on top andbottom of the inorganic layer. As long as such a layer order can beobtained, the first and second resin layers may be in direct contactwith the inorganic layer or they may be placed via undercoat layers asdescribed above to obtain tight adhesion between themselves and theinorganic layer. In the PSA sheet disclosed herein, the first resinlayer refers to the resin layer placed on the backside (the front faceof the substrate layer) of the PSA sheet relative to the inorganic layerand the second resin layer refers to the resin layer placed on the PSAlayer side.

The inorganic layer can be a metal layer formed of an aforementionedmetal material. For instance, an aluminum layer is preferable. The firstand second resin layers are preferably formed from the same material.For instance, thermoplastic resins exemplified above can be used. Ofthese materials, solely one species or a combination of two or morespecies can be used. Each of the first and second resin layers may havea layered structure with two or more layers, but is preferably amonolayer. In particular, preferable materials forming the first andsecond resin layers include PET, PP and polystyrene. PET and PP are morepreferable.

The first and second resin layers have thicknesses T_(R1) and T_(R2),respectively; and their ratio (T_(R1)/T_(R2)) is not particularlylimited, but is suitably 0.5 or greater, preferably 1 or greater, morepreferably 1.5 or greater, or yet more preferably 2.0 or greater. TheT_(R1)/T_(R2) ratio is suitably about 10 or less, preferably 7.0 orless, more preferably 5.0 or less, or yet more preferably 4.0 or less.When the T_(R1)/T_(R2) ratio is in these ranges, adherend conformabilityand crease resistance can be preferably combined. The thickness T_(R1)of the first resin layer is suitably about 10 μm or greater, preferably15 μm or greater, more preferably 18 μm or greater, or yet morepreferably 20 μm or greater (e.g. 22 μm or greater). T_(R1) is suitablyabout 100 μm or less, preferably 70 μm or less, more preferably 60 μm orless, yet more preferably 50 μm or less, or particularly preferably 35μm or less. The thickness T_(R2) of the second resin layer is suitablyabout 1 μm or greater, preferably 3 μm or greater, more preferably 5 μmor greater, or yet more preferably 7 μm or greater. T_(R2) is suitablyabout 25 μm or less, preferably 20 μm or less, more preferably 15 μm orless, or yet more preferably 12 μm or less (e.g. 10 μm or less).

The inorganic layer has a thickness T_(I) and the first and second resinlayers have a combined thickness T_(R) (=T_(R1)+T_(R2)); and their ratio(T_(R)/T_(I)) is not particularly limited. From the standpoint ofpreventing creases, protecting the inorganic layer, etc., the ratio issuitably 1 or greater, preferably 2 or greater, more preferably 3 orgreater, or yet more preferably 4 or greater. When it is bent andapplied to accommodate the adherend shape, in view of the adherendconformability, the T_(R)/T_(I) ratio is suitably 10 or less, preferably8 or less, or more preferably 6 or less. The total (T_(R)) of the firstand second resin layers' thicknesses T_(R1) and T_(R2) is suitably about15 μm or greater, preferably 20 μm or greater, more preferably 25 μm orgreater, or yet more preferably 30 μm or greater. T_(R) is suitablyabout 100 μm or less, preferably 80 μm or less, more preferably 70 μm orless, or yet more preferably 60 μm or less (e.g. 50 μm or less). Thesubstrate layer in this embodiment can effectively protect the inorganiclayer (e.g. an aluminum layer) as thin film from bending, creasing,breaking, etc. By this, even when the PSA sheet is exposed to variousstressors in the manufacturing process, etc., or even when it is exposedto a harsh environment for a long period while in use, it can securelymaintain the properties as the moisture-resistant film.

As the method for forming a laminate body having the inorganic layer,first resin layer and second resin layer, it is possible to employvarious methods as described earlier, such as a method where therespective layers are formed as films by a known method and they arelaminated dry by forming undercoat layers described above, a methodwhere the inorganic layer is formed on the first resin layer in atightly bonded manner and the second resin layer is laminated dry orextrusion-laminated on top of it, and like method.

The minimum thickness of the substrate layer is not particularlylimited. From the standpoint of the ease of manufacturing and handlingthe PSA sheet, the thickness of the substrate layer is about 3 μm orgreater, or suitably about 5 μm or greater (e.g. 10 μm or greater). Toobtain moisture resistance and rigidity unsusceptible to creasing, it isdesirable that the substrate layer is thick. From such a standpoint, thethickness of the substrate layer is preferably 15 μm or greater, morepreferably 20 μm or greater, yet more preferably 30 μm or greater, orparticularly preferably 40 μm or greater. The maximum thickness of thesubstrate layer is not particularly limited, either. It is about 1 mm orless, or suitably about 300 μm or less (e.g. 150 μm or less). From thestandpoint of the adherend conformability of the PSA sheet and ofreducing its thickness and weight, the thickness of the substrate layeris preferably 100 μm or less, more preferably 80 μm or less, yet morepreferably 70 μm or less, or particularly preferably 65 μm or less (e.g.55 μm or less). The substrate layer with such a limited thickness isless likely to lead to formation of a space between the adherend and thePSA sheet; and therefore, it can prevent water vapor permeation throughthe space.

The PSA layer side surface of the substrate layer may be subjected tocommon surface treatment, chemical or physical treatment, for instance,mattifying treatment, corona discharge treatment, crosslinkingtreatment, chromic acid treatment, ozone exposure, flame exposure,high-voltage electric shock exposure, and ionized radiation treatment.On the PSA layer-side surface of the substrate layer, an undercoat layermay be placed, which is formed by applying an undercoat such as primerto the resin layer. As the undercoat, those known in the pertinent fieldcan be used, such as urethane-based, ester-based, acrylic, andisocyanate-based kinds. From the standpoint of reducing the thicknessand weight of the PSA sheet, the thickness of the undercoat layer issuitably 7 μm or less, preferably 5 μm or less, or more preferably 3 μmor less.

<Release Liner>

In the art disclosed herein, a release liner can be used duringformation of the PSA layer; fabrication of the PSA sheet; storage,distribution and shape machining of the PSA sheet prior to use, etc. Therelease liner is not particularly limited. For example, a release linerhaving a release layer on the surface of a liner substrate such as resinfilm and paper; a release liner formed from a low adhesive material suchas a fluoropolymer (polytetrafluoroethylene, etc.) or a polyolefinicresin (PE, PP, etc.); or the like can be used. The release layer can beformed, for instance, by subjecting the liner substrate to a surfacetreatment with a release agent such as a silicone-based, long-chainalkyl-based, fluorine-based, or molybdenum disulfide-based releaseagent. When the PSA sheet is used as a sealing material for a magneticdisc device, it is preferable to use a non-silicone-based release linerfree of a silicone-based release agent which may produce siloxane gas.

<Properties of PSA Sheet>

While no particular limitations are imposed, the PSA sheet disclosedherein has a moisture permeability below 90 μg/cm² in in-plane directionof bonding interface of PSA sheet, determined at a permeation distanceof 2.5 mm over a 24-hour period based on the MOCON method(equal-pressure method). This limits moisture permeation in in-planedirections of bonding interface (vertical to the thickness direction ofthe PSA sheet) and excellent moisture resistance tends to be obtained.The moisture permeability in in-plane direction of bonding interface ispreferably below 60 μg/cm², more preferably below 30 μm/cm², or yet morepreferably below 15 μg/cm² (e.g. below 9 μg/cm²).

In particular, the moisture permeability in in-plane direction ofbonding interface is determined by the method described below.

(1) A metal plate having a 50 mm square opening at the center isobtained. FIG. 2 outlines a moisture permeability measurement device 50used for determining the moisture permeability. In FIG. 2, referencenumber 56 shows the metal plate and reference number 58 shows theopening made in metal plate 56. FIG. 3 shows a top view of metal plate56 having opening 58.(2) The PSA sheet subject to measurement is cut to a 55 mm square andapplied to cover the opening in the metal plate to prepare a measurementsample. The PSA sheet is applied to the metal plate to have a bondingwidth of 2.5 mm at each side of the opening. The PSA sheet is applied ata temperature of 23±2° C. and RH 50±10% by rolling a 2 kg roller backand forth once. The bonding width of the PSA sheet at each side of theopening is the width of the band of bonding interface between the PSAsheet and the metal plate, which is the permeation distance (mm) in anin-plane direction of bonding interface of the PSA sheet. The peripherallength of the opening in the metal plate is referred to as the bondinglength (mm) The bonding length (mm) is the total length of the band ofbonding interface exposed to water vapor. In particular, the measurementsample has a structure shown by reference number 60, formed of metalplate 56 and PSA sheet 1 applied to metal plate 56 as shown in FIG. 3.(3) Based on Method B of JIS K 7129:2008, the measurement sample isplaced between a dry chamber and a wet chamber in the moisturepermeability measurement device. In particular, as shown in FIG. 2, ameasurement sample 60 is positioned between a dry chamber 54 and a wetchamber 52. In FIG. 2, WV represents water vapor.(4) Based on the MOCON method (equal-pressure method), conditioning iscarried out for 3 hours. Subsequently, as shown in FIG. 2, at 40° C. and90% RH (relative humidity), the amount (μg) of moisture that haspermeated in an in-plane direction of bonding interface of PSA sheet perone hour is determined.(5) To obtain the moisture permeability (μg/cm²) in in-plane directionof bonding interface, the amount of moisture permeation per 24 hoursconverted from the measurement value and the PSA layer's surface area(permeation distance×bonding length) are substituted into the equation:

Moisture permeability (μg/cm²)=amount of moisture permeation(μg)/(permeation distance (cm)×bonding length (cm))

As used herein, the “moisture permeability below 90 μg/cm² in in-planedirection of bonding interface of PSA sheet, determined at a permeationdistance of 2.5 mm over a 24-hour period based on the MOCON method(equal-pressure method)” (or the “moisture permeability (μg/cm²) per24-hour measurement period in in-plane direction of bonding interface ofPSA sheet determined based on a modified MOCON method, at a permeationdistance of 2.5 mm”) can be a value obtained by a measurement over a24-hour period, but it is not limited to this; as described above, itcan be a 24-hour value converted from a measurement taken for a certaintime period (e.g. one hour). The measurement time can be longer than onehour (preferably about 6 hours; the same applies to working examplesdescribed later) and the value per 24 hours converted from thismeasurement value can be used as well.

The kind of metal plate is not particularly limited. For instance, analuminum plate can be used. The size of the metal plate is notparticularly limited, either. In accordance with the size of testingdevice, etc., for instance, a 100 mm square plate can be used. It issuitable to use a metal plate having a smooth surface, for instance, onehaving a mean arithmetic roughness Ra of about 3 μm or less. Inparticular, an aluminum A1050 plate (0.3 mm thick, surface roughness:mirror finished, Ra 0.1 μm) is used. As the testing device, product namePERMATRAN-W3/34G available from MOCON, Inc. or a comparable product canbe used. In a testing device of this type, N₂ gas at 90% RH can besupplied to the wet chamber and N₂ gas at 0% RH can be supplied to thedry chamber. This maintains the two chambers divided by the measurementsample at an equal pressure. For the measurement, the gas flow is set at10 mL/min. In the testing device, the water vapor concentration ismeasured by an infrared sensor (indicated as “IR” in FIG. 8), but themeans of detection is not limited to this. The position of themeasurement sample in the testing device is not particularly limited.The adhesive face of the PSA sheet can be placed either on the wetchamber side or on the dry chamber side. The same measurement method isemployed in the working examples described later. The measurement valueof moisture permeation is used as the amount (μg) of moisture permeatedupon zero-point correction based on the measurement value of an aluminumplate with no opening. The same measurement method is employed in theworking examples described later.

The measurement method has been created by the present inventors. Thismethod enables previously impossible, highly precise quantification ofmoisture permeation in in-plane direction of bonding interface. Morespecifically, differences in moisture permeability in in-plane directionof bonding interface can be detected as significant differences amongdifferent samples that have shown similar values in quantification ofmoisture permeation by conventional cup methods. By employing thismethod, moisture resistance can be tested at a higher level. Forinstance, it enables quantification of water vapor permeation that is ina minute amount, yet still capable of affecting HAMR.

While no particular limitations are imposed, the PSA sheet disclosedherein preferably has an amount of thermally released gas of 10 μg/cm²or less (in particular, 0 to 10 μg/cm²) when determined at 130° C. for30 minutes by GC-MS. The PSA sheet with such highly-limited thermal gasrelease can be preferably used in an application (typically a magneticdisc device) for which the presence of volatile gas is undesirable. Whenthe PSA sheet satisfying this property is used as a sealing material fora magnetic disc device, it can highly inhibit internal contaminationwith siloxane and other gas that affect the device. The amount ofthermally released gas is preferably 7 μg/cm² or less, more preferably 5μg/cm² or less, yet more preferably 3 μg/cm² or less, or particularlypreferably 1 μg/cm² or less.

The amount of thermally released gas is determined based on the dynamicheadspace method. In particular, a PSA sheet subject to measurement iscut out to a 7 cm² size to obtain a measurement sample. The measurementsample is sealed in a 50 mL vial and heated at 130° C. for 30 minutes,using a headspace autosampler. As the headspace autosampler, acommercial product can be used without particular limitations. Forinstance, product name EQ-12031HSA available from JEOL Ltd., or acomparable product can be used. The total amount of gas released fromthe measurement sample is determined by gas chromatography/massspectrometry (GC-MS). A commercial GC-MS can be used. The amount ofthermally released gas is the amount of gas released per unit surfacearea of PSA sheet (in μg/cm²). The same measurement method is employedin the working examples described later.

The PSA sheet disclosed herein has a 180° peel strength to stainlesssteel (an adhesive strength) of preferably 3 N/20 mm or greater, whendetermined based on JIS Z 0237:2009. Having such an adhesive strength,the PSA sheet can bond well to an adherend to provide good sealing. Theadhesive strength is more preferably 5 N/20 mm or greater, yet morepreferably 8 N/20 mm or greater, or particularly preferably 10 N/20 mmor greater (e.g. 12 N/20 mm or greater). The maximum adhesive strengthis not particularly limited. From the standpoint of preventing left-overadhesive residue, it is suitably about 20 N/20 mm or less (e.g. about 15N/20 mm or less).

The adhesive strength of a PSA sheet is determined by the followingmethod: A PSA sheet subject to measurement is cut to a 20 mm wide, 100mm long size to prepare a test piece. In an environment at 23° C. and50% RH, the adhesive face of the test piece is press-bonded to astainless steel plate (SUS304BA plate) to obtain a measurement sample.The press-bonding is carried out by rolling a 2 kg roller back and forthonce. The measurement sample is left standing in an environment at 23°C. and 50% RH for 30 minutes. Subsequently, using a tensile tester,based on JIS Z 0237:2009, the peel strength (N/20 mm) is determined at atensile speed of 300 mm/min at a peel angle of 180°. As the tensiletester, Precision Universal Tensile Tester Autograph AG-IS 50N availablefrom Shimadzu Corporation or a comparable product can be used. The samemeasurement method is employed in the working examples described later.

The PSA sheet disclosed herein preferably shows a displacement less than2 mm in a shear holding power test carried out with a 1 kg load at 60°C. for one hour. The PSA sheet satisfying this property shows goodholding power even when used at a relatively high temperature. Thedisplacement in the shear holding power test is more preferably lessthan 1 mm, or yet more preferably less than 0.7 mm (e.g. less than 0.5mm, or even less than 0.1 mm) The PSA sheet according to a particularlypreferable embodiment shows no displacement (i.e. a displacement ofabout 0 mm) in the shear holding power test.

The shear holding power of a PSA sheet is determined by the followingmethod: In particular, the PSA sheet subject to measurement is cut 10 mmwide, 20 mm long to prepare a test piece. In an environment at 23° C.and 50% RH, the adhesive face of the test piece is press-bonded to astainless steel plate to obtain a measurement sample. The press-bondingis carried out by rolling a 2 kg roller back and forth once. Themeasurement sample is vertically suspended and left in an environment at60° C. and 50% RH for 30 minutes. Subsequently, a 1 kg weight isattached to the free lower end of the test piece to start the test. Thetest is carried out for one hour and the distance that the test piecedisplaced (the displacement) is measured at one hour. The samemeasurement method is employed in the working examples described later.

The PSA sheet disclosed herein preferably has a tensile modulus per unitwidth in a prescribed range. In particular, the tensile modulus ispreferably greater than 1000 N/cm, more preferably greater than 1400N/cm, yet more preferably greater than 1800 N/cm, or particularlypreferably greater than 2200 N/cm. The PSA sheet having such a tensilemodulus has suitable rigidity and is less susceptible to creasing. Ittends to provide excellent handling properties as well. The tensilemodulus is preferably less than 3500 N/cm, more preferably less than3000 N/cm, or yet more preferably less than 2800 N/cm (e.g. less than2600 N/cm). The PSA sheet having such a tensile modulus has goodadherend conformability and can well conform in a bent state to an areaof the adherend including a corner.

The tensile modulus per unit width of PSA sheet is determined asfollows: In particular, the PSA sheet is cut to a 10 mm wide, 50 mm longstrip to prepare a test piece. The two ends of the length of the testpiece are clamped with chucks in a tensile tester. In an atmosphere at23° C., at an inter-chuck distance of 20 mm, at a speed of 50 mm/min, atensile test is conducted using the tensile tester to obtain astress-strain curve. Based on the initial slope of the resultingstress-strain curve, the Young's modulus (N/mm²=MPa) is determined bylinear regression of the curve between two specified strain points ε1and ε2. From the product of the resulting value and the thickness of thePSA sheet, the tensile modulus per unit width (N/cm) can be determined.As the tensile tester, a commonly known or conventionally used productcan be used. For instance, AUTOGRAPH AG-IS available from ShimadzuCorporation or a comparable product can be used.

The total thickness of the PSA sheet disclosed herein is notparticularly limited. It is suitably about 6 μm or greater. From thestandpoint of the moisture resistance and crease resistance, etc., it ispreferably 25 μm or greater, more preferably 40 μm or greater, or yetmore preferably 60 μm or greater. The total thickness is suitably about12 mm or less. From the standpoint of the adherend conformability and ofreducing the thickness and weight, it is preferably 200 μm or less, morepreferably 150 μm or less, or yet more preferably 120 μm or less (e.g.less than 100 μm). The total thickness of a PSA sheet here refers to thecombined thickness of the substrate layer and the PSA layer, notincluding the thickness of the release liner described later.

<Applications>

The PSA sheet disclosed herein has excellent moisture resistance, and ina preferable embodiment, gas emission is reduced; and therefore, it ispreferably used in various applications where entry of moisture (andentry of gas if necessary) is desirably limited. For instance, the PSAsheet disclosed herein is preferably used in various electronic devices.More specifically, it is preferably used as a blocking material in theelectronic devices (e.g. a sealing material to seal their internalspaces). In a more preferable embodiment, for instance, the PSA sheet isused for sealing the internal space of a magnetic disc device such asHDD. In this application, an included gas such as siloxane gas may causedamage to the device; and therefore, it is desirable to prevent such gascontamination. In a magnetic disc device employing HAMR, it is importantto prevent entrance of water which badly affects the recording life. Byusing the PSA sheet disclosed herein as a sealing material (or a coverseal) for a HAMR magnetic disc device, a magnetic recording devicehaving a higher density can be obtained.

FIG. 4 shows an embodiment of the magnetic disc device as a favorableexample to which the art disclosed herein can be applied. FIG. 4 shows across-sectional diagram schematically illustrating the magnetic discdevice according to an embodiment. A magnetic disc device 100 comprisesa data-recording magnetic disc 110, a spindle motor 112 that rotatesmagnetic disc 110, a magnetic head 114 that reads and writes data onmagnetic disc 110, and an actuator 116 that supplies power to magnetichead 114. Actuator 116 has a built-in linear motor not shown in thedrawing. In this example of constitution, two magnetic discs 110 areincluded, but it is not limited to this and three or more magnetic discsmay be included.

These components of magnetic disc device 100 are placed in a housing 120which serves as a casing for magnetic disc device 100. In particular,the components of magnetic disc device 100 are contained in a box-shapedhousing base member (a support structure) 122 having a top opening andthe top opening of housing base member 122 is covered with a rigid covermember 124. More specifically, the top opening of housing base member122 has a recessed portion around the inner circumference and the outerrim of cover member 124 is placed on the bottom of recessed portion 126,with cover member 124 covering the opening. A PSA sheet 101 is appliedfrom the top face of cover member 124 so as to entirely cover the covermember 124 and the top face (outer circumference of the opening) ofhousing 120, that is, the entire top face of housing 120, altogether.This seals a space 140 present between housing base member 122 and covermember 124 as well as other holes and void spaces that communicate fromthe inside to the outside of magnetic disc device 100, thereby keepingthe inside of the device air-tight. Such a sealing structure using PSAsheet 101 as the sealing material (cover seal) can be made thinner thana conventional counterpart that uses a cover member and a gasket toobtain air-tight properties. In addition, because it does not requirethe use of a liquid gasket, outgassing from the gasket can be eliminatedas well. In this embodiment, the width of the top rim (face of theframe) of housing base member 122 is about 0.1 mm to 5 mm (e.g. 3 mm orless, or even 2 mm or less) at its narrowest portion, with the widthbeing the distance between the outer circumference and innercircumference of the top rim of housing base member 122. When PSA sheet101 is applied as a cover seal to the top face of housing base member122, the top rim of housing base member 122 provides a bonding surfaceto PSA sheet 101, forming a portion that isolates the internal space ofmagnetic disc device 100 from the outside. According to the artdisclosed herein, even in an application where the width of bondingsurface (through-bonding-plane permeation distance) is limited, theinternal space can be maintained air-tightly and dry(moisture-resistant).

FIG. 5 shows another embodiment of the magnetic disc device to which theart disclosed herein can be applied. A magnetic disc device 200 hasbasically the same constitution as the embodiment described above exceptfor the way a PSA sheet 201 is applied. Different features are describedbelow. In magnetic disc device 200, PSA sheet 201 covers cover member224 and the top face (outer circumference of the opening) of housingbase member 222 altogether, having a margin (or an extending portion)that further extends to the side of housing 220. In particular, theextending portion is bent from the top face over the corner of top rimto the side of housing base member 222. The extending portion may beprovided entirely or partially at each side forming the outercircumference of the top face of housing 220. In other words, inmagnetic disc device 200, PSA sheet 201 is applied, at least partiallycovering the top and side faces of housing 220 in a U shape. Similar toPSA sheet 101 according to the embodiment described above, PSA sheet 201seals a space 240 present between housing base member 222 and covermember 224 as well as other holes and void spaces that communicate fromthe inside to the outside of magnetic disc device 200; and because it isapplied with a margin extending to the side of housing base member 222,the sealed state is extended in the in-plane directions of bondinginterface. This results in a larger distance (width) of the bondinginterface of PSA sheet 201 separating the outside and space 240, etc.,and it inhibits moisture permeation via the bonding interface of PSAsheet 201, thereby further enhancing the moisture resistance. In thisembodiment, the distance of PSA sheet 201 extending from the top rim(top edge of the side) to the side of housing 220 (i.e. the length ofPSA sheet 201 that covers the side (lateral surface)) is about 1 mm orgreater (e.g. 2 mm or greater, or even 3 mm or greater).

In these embodiments, cover members 124 and 224 cover magnetic discs 110and 210 as well as actuators 116 and 216 altogether, respectively, inone piece. However, they are not limited to these. They may covermagnetic discs 110 and 210, actuators 116 and 216, and other components,separately; or they may not cover actuators 116 or 216 while coveringmagnetic discs 110 and 210. Even in these embodiments, by applying thePSA sheet over the cover member, the inside of the device can be mademoisture-resistant and air-tight. In a magnetic disc device having suchan embodiment, the moisture resistance and air-tight properties areobtained with the thin PSA sheet, thereby achieving a thin sealingstructure. This can increase the capacity for housing magnetic discs,bringing about a magnetic disc device having a higher density and alarger capacity.

Matters disclosed by this description include the following:

(1) A magnetic disc device comprising

at least one data-recording magnetic disc,

a motor that rotates the magnetic disc,

a magnetic head that at least either reads or writes data on themagnetic disc,

an actuator that moves the magnetic head, and

a housing that encases the magnetic disc, the motor, the magnetic headand the actuator; wherein

the housing is provided with a cover seal, the cover seal being a PSAsheet, and

the PSA sheet has a PSA layer comprising a polymer A and a polymer Bdifferent from the polymer A, with isobutylene polymerized in each ofthe polymer A and the polymer B, accounting for 50% by weight or morethereof (i.e. the polymer A and the polymer B are individually formedwith at least 50% (by weight) polymerized isobutylene).

(2) The magnetic disc device according to (1) above, wherein the housingcomprises a box-shaped housing base member having a top opening and acover member to cover the opening.(3) The magnetic disc device according to (2) above, wherein the housingbase member has a recessed portion around the inner circumference of thetop opening and the outer rim of the cover member is placed on thebottom of the recessed portion.(4) The magnetic disc device according to any of (1) to (3) above,wherein the cover member has a hole.(5) The magnetic disc device according to any of (1) to (4) above,wherein the PSA sheet seals the internal space of the magnetic discdevice.(6) The magnetic disc device according to any of (1) to (5) above,wherein the PSA sheet covers and seals the top face of the housing basemember of the magnetic disc device.(7) The magnetic disc device according to any of (1) to (6) above,capable of heat-assisted magnetic recording.(8) The magnetic disc device according to any of (1) to (7) above,wherein the PSA layer has a storage modulus below 0.5 MPa at 25° C.(9) The magnetic disc device according to any of (1) to (8) above,wherein, in addition to the isobutylene, isoprene is copolymerized inthe polymer A.(10) The magnetic disc device according to any of (1) to (9) above,wherein the polymer A and the polymer B has a combined amount thataccounts for 90% by weight or more of the PSA layer.(11) A PSA composition comprising a polymer A and a polymer B differentfrom the polymer A, wherein

in each of the polymer A and the polymer B, isobutylene is polymerizedat a ratio of 50% by weight or higher.

(12) The PSA composition according to (11) above, wherein, in additionto the isobutylene, isoprene is copolymerized in the polymer B.(13) The PSA composition according to (11) or (12) above, wherein thepolymer A has a weight average molecular weight in the range between1×10⁴ and 80×10⁴.(14) The PSA composition according to any of (11) to (13) above, whereinthe polymer B has a weight average molecular weight in the range between5×10⁴ and 150×10⁴.(15) The PSA composition according to any of (11) to (14) above, whereinthe polymer A has a weight average molecular weight M_(A) and thepolymer B has a weight average molecular weight M_(B), with aM_(B)/M_(A) ratio value in the range between 5 and 100.(16) The PSA composition according to any of (11) to (15), having aweight ratio (C_(A)/C_(B)) of the polymer A content C_(A) to the polymerB content C_(B) in the range between 70/30 and 30/70.(17) The PSA composition according to any of (11) to (16) above, whereinthe polymer A and the polymer B has a combined amount accounting for 90%by weight or more of solid content of the PSA composition.(18) The PSA composition according to any of (11) to (17) above, whereinthe isobutylene has a copolymerization ratio of 90% by weight or abovein the polymer A.(19) The PSA composition according to any of (11) to (18) above, whereinthe polymer A is polyisobutylene and the polymer B is butyl rubber.(20) The PSA composition according to any of (11) to (19) above, usedfor sealing an internal space of a magnetic disc device.(21) A PSA sheet having a PSA layer comprising a polymer A and a polymerB different from the polymer A, wherein

in each of the polymer A and the polymer B, isobutylene is polymerizedat a ratio of 50% by weight or above.

(22) The PSA sheet according to (21) above, wherein, in addition to theisobutylene, isobutylene and isoprene are copolymerized in the polymerB.(23) The PSA sheet according to (21) or (22) above, wherein the polymerA has a weight average molecular weight in the range between 1×10⁴ and80×10⁴.(24) The PSA sheet according to any of (21) to (23) above, wherein thepolymer B has a weight average molecular weight in the range between5×10⁴ and 150×10⁴.(25) The PSA sheet according to any of (21) to (24) above, wherein thepolymer A has a weight average molecular weight M_(A) and the polymer Bhas a weight average molecular weight M_(B), with a M_(B)/M_(A) ratiovalue in the range between 5 and 100.(26) The PSA sheet according to any of (21) to (25) above, having aweight ratio (C_(A)/C_(B)) of the polymer A content C_(A) to the polymerB content C_(B) in the range between 70/30 and 30/70.(27) The PSA sheet according to any of (21) to (26) above, wherein thepolymer A and the polymer B has a combined amount accounting for 90% byweight or more of solid content of the PSA composition.(28) The PSA sheet according to any of (21) to (27) above, wherein thePSA layer has a storage modulus at 25° C., G′(25° C.), of 0.09 MPa orgreater and 0.29 MPa or less.(29) The PSA sheet according to any of (21) to (28) above, used forsealing an internal space of a magnetic disc device.(30) The PSA sheet according to any of (21) to (29) above, used forsealing an internal space of a magnetic disc device capable ofheat-assisted magnetic recording.(31) The PSA sheet according to any of (21) to (30) above, having amoisture permeability of less than 90 μg/cm² in in-plane direction ofbonding interface of PSA sheet, measured based on the MOCON method, at apermeation distance of 2.5 mm over a 24-hour period.(32) The PSA sheet according to any of (21) to (31) above, having anamount of thermally released gas of 10 μg/cm² or less, determined at130° C. for 30 minutes by gas chromatography/mass spectrometry.(33) The PSA sheet according to any of (21) to (32) above, having a 180°peel strength to stainless steel plate of 3 N/20 mm or greater.(34) The PSA sheet according to any of (21) to (33) above, wherein thePSA layer has a storage modulus less than 0.5 MPa at 25° C.(35) The PSA sheet according to any of (21) to (34) above, showing adisplacement less than 2 mm in a shear holding power test carried outwith a 1 kg load at 60° C. for one hour.(36) The PSA sheet according to any of (21) to (35) above, having atensile modulus per unit width above 1000 N/cm and below 3500 N/cm.(37) The PSA sheet according to any of (21) to (36) above, having atotal thickness of 25 am to 200 am.(38) The PSA sheet according to any of (21) to (37) above, furthercomprising a substrate layer (moisture-impermeable layer), with the PSAlayer provided to one face of the substrate layer.(39) The PSA sheet according to any of (21) to (38) above, the substratelayer includes an inorganic layer.(40) The PSA sheet according to (39) above, wherein the inorganic layeris a metal layer.(41) The PSA sheet according to (39) or (40) above, wherein theinorganic layer is formed of aluminum or an aluminum alloy(42) The PSA sheet according to any of (39) to (41) above, wherein theinorganic layer has a thickness of 2 am to 20 am.(43) The PSA sheet according to any of (39) to (42) above, wherein thesubstrate layer comprises a resin layer in addition to the inorganiclayer.(44) The PSA sheet according to (43) above, wherein the resin layer is apolyester resin layer.(45) The PSA sheet according to (43) or (44) above, wherein the resinlayer has a thickness of 3 μm to 55 μm.(46) The PSA sheet according to any of (38) to (45) above, wherein thesubstrate layer is formed of a laminate comprising an inorganic layer aswell as first and second resin layers laminated atop and below theinorganic layer.(47) A release-linered PSA sheet, comprising the PSA sheet according toany of (21) to (46) above and a release liner protecting the adhesiveface of the PSA sheet, wherein the release liner is a non-silicone-basedrelease liner free of a silicone-based release agent.(48) A magnetic disc device comprising the PSA sheet according to any of(21) to (46) above.(49) The magnetic disc device according to (48) above, wherein the PSAsheet seals the internal space of the magnetic disc device.(50) The magnetic disc device according to (48) or (49) above, whereinthe magnetic disc device has a housing base member and the PSA sheet isa cover seal that covers and seals the top face of the housing basemember.(51) The magnetic disc device according to any of (48) to (50), capableof heat-assisted magnetic recording.

Several working examples related to the present invention are describedbelow, but the present invention is not intended to be limited to theseexamples. In the description below, “parts” and “%” are by weight unlessotherwise specified.

Example 1

By dry bonding lamination, were laminated 25 μm thick PET film (PETlayer) as the first resin layer, 7 μm thick aluminum foil (Al layer) asthe inorganic layer and 9 μm thick PET film (PET layer) as the secondresin layer in this order from the front (outer surface side) to thebackside (PSA layer side). Between each resin layer and the inorganiclayer, was laminated a 3 μm thick adhesive layer. A 47 μm thicksubstrate layer (moisture-impermeable layer) was thus prepared.

In toluene, were dissolved Polyisobutylene A (PIRA: product name OppanolN50 available from BASF Corporation, Mw˜34×10⁴, Mw/Mn 5.0) and butylrubber (IIR: product name JSR BUTYL 268 available from JSR Corporation,Mw˜54×10⁴, Mw/Mn˜˜4.5, 98.3 mol % isobutylene, 1.7 mol % isoprene) at50:50 blend ratio to prepare a PSA composition with 25% NV. The PSAcomposition obtained above was applied to one face (the second resinlayer-side surface) of the substrate layer to have a thickness of 30 μmafter dried, and allowed to dry at 120° C. for 3 minutes to form a PSAlayer. A PSA sheet was thus obtained according to this Example. Forprotection of the surface (adhesive face) of the PSA layer, was used arelease liner formed of thermoplastic film treated with release agent(product name HP-S0 available from Fujico Co., Ltd.; 50 μm thick).

Example 2

Using a 50:50 mixture of Polyisobutylene B (PIB-B: product name OppanolN80 available from BASF Corporation, Mw˜75×10⁴, Mw/Mn 5.0) and IIR, butotherwise in the same manner as Example 1, was prepared a PSAcomposition. Using the resulting PSA composition, in the same manner asExample 1, was obtained a PSA sheet according to this Example.

Example 3

Using a 50:50 mixture of Polyisobutylene C (PIB-C: product name OppanolB15 available from BASF Corporation, Mw˜7.5×10⁴, Mw/Mn 5.0) and IIR, butotherwise in the same manner as Example 1, was prepared a PSAcomposition. Using the resulting PSA composition, in the same manner asExample 1, was obtained a PSA sheet according to this Example.

Example 4 to 5

The blend ratio of PIB-C and IIR was changed to the ratios shown inTable 1. Otherwise in the same manner as Example 3, were obtained PSAsheets according to the respective Examples.

Example 6

In toluene, was dissolved PIB-A to prepare a PSA composition with 25%NV. Using this PSA composition, but otherwise in the same manner asExample 1, was prepared a PSA composition. Using the resulting PSAcomposition, in the same manner as Example 1, was obtained a PSA sheetaccording to this Example.

Example 7

Using PIB-B, but otherwise in the same manner as Example 6, was prepareda PSA composition. Using the resulting PSA composition, in the samemanner as Example 1, was obtained a PSA sheet according to this Example.

Example 8

Using PIB-C, but otherwise in the same manner as Example 6, was prepareda PSA composition. Using the resulting PSA composition, in the samemanner as Example 1, was obtained a PSA sheet according to this Example.

Example 9

Using IIR, but otherwise in the same manner as Example 6, was prepared aPSA composition. Using the resulting PSA composition, in the same manneras Example 1, was obtained a PSA sheet according to this Example.

[Moisture Permeability (Cup Method) of PSA Layer]

The moisture permeability in the thickness direction of each PSA layerwas determined based on the water vapor permeability test (cup method)in JIS Z 0208. In particular, the PSA composition was applied to areleasable surface and allowed to dry to form a 50 μm thick PSA layer.The PSA layer was adhered to 2 μm thick PET film (DIAFOIL available fromMitsubishi Plastics, Inc.) with a rubber roller. The PET layer-supportedPSA layer was cut to a circle of 30 mm diameter to fit the diameter ofthe test cup (an aluminum cup of 30 mm diameter used in the cup methodof JIS Z 0208). This was used as a test sample. A prescribed amount ofcalcium chloride was placed in the cup and the opening of the cup wassealed with the test sample prepared above. The cup covered with thetest sample was placed in a thermostat wet chamber at 60° C. and 90% RHand left standing for 24 hours. The change in weight of calcium chloridebefore and after this step was determined to obtain the moisturepermeability (g/cm²·24 h).

For each Example, Table 1 summarizes the PSA and shows the test resultsof moisture permeability (cup method) (g/cm²·24 h), storage moduliG′(25° C.) (MPa), through-bonding-plane moisture permeability of PSAsheet (μg/cm²), adhesive strength (N/20 mm), shear holding power (mm),and amount of thermally released gas (μg/cm²).

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 PSAPolymer A Species PIB PIB PIB PIB PIB PIB PIB PIB Mm (×10⁴) 34 75 7.57.5 7.5 34 75 7.5 Polymer B Species IIR IIR IIR IIR IIR IIR Mw (×10⁴) 5454 54 54 54 54 Polymer A/Polymer B 50/50 50/50 50/50 60/40 80/20 100/0100/0 100/0 0/100 PSA layer's elastic modulus @25° C. (MPa ) 0.18 0.240.16 0.24 0.16 0.28 0.30 0.30 0.25 Moisture permeability (cup method)(g/cm² · 24 h) 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 Moisture permeabilityin in-plane direction 7 10 5 14 16 28 41 19 30 of bonding interface(μg/cm²) Adhesive strength (N/20 mm) 11 6.2 13 7.6 5.6 5.5 1.6 10.0 2.6Shear holding power (mm) 0.5 0.2 0.5 0.6 0.8 0.3 0.2 3.0 0.2 Amount ofthermally released gas (μg/cm²) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.3 0.8

As shown in Table 1, with respect to the moisture permeability test bythe cup method, no differences were observed among Examples 1 to 9;however, when the moisture permeability in in-plane direction of bondinginterface was precisely tested, there were notable differences among therespective Examples. In particular, with respect to the PSA sheetsaccording to Examples 1 to 5 comprising polymers A and B, the moisturepermeability in in-plane direction of bonding interface had a tendencyto decrease as compared to Examples 6 to 9. These results indicate thatexcellent moisture resistance can be obtained with a composition wherethe polymers A and B are used together.

Although specific embodiments of the present invention have beendescribed in detail above, these are merely for illustrations and do notlimit the scope of claims. The art according to the claims includesvarious modifications and changes made to the specific embodimentsillustrated above.

REFERENCE SIGNS LIST

-   1, 101, 201 PSA sheets-   10 substrate layer-   12 first resin layer-   14 inorganic layer-   16 second resin layer-   20 PSA layer-   50 moisture permeability measurement device-   52 wet chamber (first chamber)-   54 dry chamber (second chamber)-   56 metal plate (partition plate)-   58 opening-   60 measurement sample-   100, 200 magnetic disc devices-   110, 210 magnetic discs-   112, 212 spindle motors-   114, 214 magnetic heads-   116, 216 actuator-   120, 220 housing-   122, 222 housing base member-   124, 224 cover member-   126, 226 recessed portions-   140, 240 spaces

1. A pressure-sensitive adhesive composition comprising a polymer A anda polymer B different from the polymer A, wherein in each of the polymerA and the polymer B, isobutylene is polymerized at a ratio of 50% byweight or higher.
 2. The pressure-sensitive adhesive compositionaccording to claim 1, wherein, in addition to the isobutylene, isopreneis copolymerized in the polymer B.
 3. The pressure-sensitive adhesivecomposition according to claim 1, wherein the polymer A has a weightaverage molecular weight in the range between 1×10⁴ and 80×10⁴.
 4. Thepressure-sensitive adhesive composition according to claim 1, whereinthe polymer B has a weight average molecular weight in the range between5×10⁴ and 150×10⁴.
 5. The pressure-sensitive adhesive compositionaccording to claim 1, wherein the polymer A has a weight averagemolecular weight M_(A) and the polymer B has a weight average molecularweight M_(B), with a M_(B)/M_(A) ratio value in the range between 5 and100.
 6. The pressure-sensitive adhesive composition according to claim1, having a weight ratio (C_(A)/C_(B)) of polymer A content C_(A) topolymer B content C_(B) (a weight ratio of the polymer A to the polymerB) in the range between 70/30 and 30/70.
 7. The pressure-sensitiveadhesive composition according to claim 1, wherein the polymer A and thepolymer B has a combined amount accounting for 90% by weight or more ofsolid content of the pressure-sensitive adhesive composition.
 8. Thepressure-sensitive adhesive composition according to claim 1, used forsealing an internal space of a magnetic disc device.
 9. Apressure-sensitive adhesive sheet having a pressure-sensitive adhesivelayer comprising a polymer A and a polymer B different from the polymerA, wherein in each of the polymer A and the polymer B, isobutylene ispolymerized at a ratio of 50% by weight or above.
 10. Thepressure-sensitive adhesive sheet according to claim 9, wherein thepressure-sensitive adhesive layer has a storage modulus at 25° C.,G′(25° C.), of 0.09 MPa or greater and 0.29 MPa or less.